a friend has an '84 300td and he can't get any heat through the floor vents under the dash.he can get heat at the panel vents and defrost but not at the floor.i told him it sounds like a vacuum problem with the directional doors.i did a seach and found a mention of a valve misfunction-monovalve.where is this valve and could it have anything to do with his heat?

__________________
'95 s500 (bought for wife but can't bear to share!!!) 125kms
'92 legend 180kms
'88 tbirdturbo(fantastic car-only regular maint.)120kms
'87 mustang gt(gone)
'86 tbirdturbo(gone)
'85 mustang gt(gone-but not forgotten)